In order to realize an image search on the Internet, it is necessary to associate each image with a description that describes the image. However, as an enormous number of images exist on the Internet, it is extremely difficult to manually associate each image with a description.
In view of this, Google Inc. provides a service called “Google Image Labeler,” which allows many users to play a game of matching a specific image to a description, and obtains a description that describes the specific image using the game results (see, for example, Non-Patent Document 1).
Specifically, this game presents the same image to two players, asks the players to input as many descriptions as possible that appropriately describe the image, and gives a score to both players when the players' descriptions match (see, for example, Patent Document 1). The more each player plays the game to earn a high score, the larger the number of images associated with descriptions, and the higher the precision of an image search.